Hybrid vehicles are driven by multiple powerplants including, but not limited to an internal combustion engine and an electric machine. The electric machine functions as a motor/generator. In a generator mode, the electric machine is driven by the engine to generate electrical energy used to power electrical loads or charge batteries. In a motor mode, the electric machine supplements the engine, providing drive torque to drive the vehicle drivetrain.
When the hybrid vehicle is at rest and no drive torque is required, the engine is deactivated. Vehicle launch is initiated by the operator in one of several manners including, but not limited to, depressing an accelerator pedal and releasing pressure from a brake pedal. Prior to activating the engine, the electric machine spins up or drives the engine to a desired idle speed. As the engine spins up to idle speed, compression torque induces speed sag. This causes resonance within the powertrain, which can be sensed by the operator.